The basic meaning of the word waseelah is a path that brings one near to something.
Allah commands His believing slaves to fear Him (taqwa). When this word is accompanied by mention of obedience, it means refraining from haram things.
It says that Seeking nearness to Allah is done through Obedience and leaving the sins.
After that Allah says, "And seek the means of approach to Him".
What is meant by waseelah here is drawing close to Allah by obeying His commands and avoiding that which He has forbidden, in accordance with the teachings brought by Muhammad (peace and blessings of Allah be upon him).(mofradat,page561)
So while the word waseelah is a path that brings one near to Allah, It includes:
- Obedience of Allah
- avoiding sins
- jihad for Allah
- Sincerity (Ikhlas)
- visiting relatives(sele rahem)
- Imams and Ahlul Bayt
Specially Ahadith interpret waseelah to Imams. The Prophet (PBUH) said:
الائمة من ولد الحسین(ع) من اطاعهم فقد اطاع اللّه و من عصاهم فقد عصى
اللّه هم العروة الوثقى و هم الوسیلة الى اللّه تعالى"
Imams are the children of Imam Hussein (A.S.) which who obey
them, obeyed Allah; And who disobey them, disobeyed Allah. They are the
most trustworthy handhold And they are waseelah for nearness to
Allah.(Nour Alsaghalain,volume 1,page 624)
In Interpretation of Ali ibn Ibrahim,he interprets this verse as:
"تقرّبوا الیه بالامام"
Seek Approach to Allah by Imams.
Also in hadith saqlain narrated that adherence to Quran and Ahlul Bayt is a waseelah to nearness to Allah.