On tawakkul
First of all having trust or confidence in Allah is one of the major and essential parts of monotheism.
As this means that you do a thing and believe that it is in the hands of Allah to cause or bring any benefit and throw away any bear.
Allah the almighty says:
... And whoever relies upon Allah - then He is sufficient for him. Indeed, Allah will accomplish His purpose. Allah has already set for everything a [decreed] extent. (65:3)
Therefore the prophet () has taught ibn 'Abbas saying:
'O boy! I will teach you a statement: Be mindful of Allah and He will protect you. Be mindful of Allah and you will find Him before you. When you ask, ask Allah, and when you seek aid, seek Allah's aid. Know that if the entire creation were to gather together to do something to benefit you- you would never get any benefit except that Allah had written for you. And if they were to gather to do something to harm you- you would never be harmed except that Allah had written for you. The pens are lifted and the pages are dried.'
(Jami' at-Tirmdihi and according to ibn Kathir also in Musnad Ahmad -see in here qtafsir-)
Many verses of the qur'an show the basis of tawakkul (trust and reliance in Allah) for example repeat every day at least seventeen times:
It is You we worship and You we ask for help. (1:5)
It is an attribute of the true believers:
... and upon Allah the believers should rely. (3:122)
The believers are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely - (8:2)
... But what is with Allah is better and more lasting for those who have believed and upon their Lord rely (42:36)
And Allah reminds us:
... so worship Him and rely upon Him. ... (11:123)
On the interpretation of "tie it and trust in Allah"
In his tohfat al-Awhwadhi تحفة الأحوذي al-Mubrakpuri محمد بن عبد الرحمن بن عبد الرحيم المباركفوري when commenting on this hadith said:
In the following I will translate from Arabic, as it is my own translation take it carefully
( قال اعقلها ) قال المناوي : أي شد ركبة ناقتك مع ذراعيها بحبل ( وتوكل ) أي اعتمد على الله ، وذلك لأن عقلها لا ينافي التوكل
(He said: "Tie it) means tie the knee with the cubit with a rope (and trust in Allah) means rely on Allah, this is because tying the camel is not opposed to tawakkul (trust in Allah).
On tawakkul and taking means and measures
Trust in Allah doesn't mean that we have to leave things to Allah without taking means and measures. The prophets () are good examples when it comes to trust in Allah nevertheless they also taught us to take means and measures Nuh () built the ark, Yusuf () didn't tell his dream to his brothers, they followed the instruction of their father not to enter from the same door ... Allah says:
... And when you have decided, then rely upon Allah . Indeed, Allah loves those who rely [upon Him]. (3:159)
So we may take a decision, make a plan take means and measures and then trust in Allah and do whatever we have planed before.
Allah also say:
O you who have believed, take your precaution and [either] go forth in companies or go forth all together. (4:71)
And prepare against them whatever you are able of power and of steeds of war ... (8:60)
which generally means be prepared. And if our taken means and measures are not sufficient we are taught that Allah is helping his true believers:
And when the two companies saw one another, the companions of Moses said, "Indeed, we are to be overtaken!" (61) [Moses] said, "No! Indeed, with me is my Lord; He will guide me." (62) (26:61-62)
and we know how the story ended.
As good believers we may also remind ourselves verses such as:
... But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not. (2:216)
And you do not will except that Allah wills - Lord of the worlds. (81:29)
I intended this as a short addition to the good answer of @TheZ.
As for the guidelines of taking means and measures red the fatwa islamqa #213652
