What does " On a certain level " in " I am looking for my sister ... on a certain level" mean ? I have read it in a play.I checked it in some dictionaries but coulden't find its meaning.
The phrase on a certain level is generally used to mean to a limited degree.
In the original example, " I am looking for my sister ... on a certain level", the suggestion is that the speaker is not fully engaged in the effort, but is doing so reluctantly or half-heartedly.
On a certain level, we want the Congress to continue its intransigence. Maybe the electorate will see them for what they are and throw the bums out.
In the above example, in large part, the speaker does not like the intransigence and wants it to end. But she also recognizes that the unwanted activity might have some side benefit. She has mixed feelings and recognizes that a small benefit may come from a generally bad situation.
It also is used to deny the applicability of the characterization on all other levels.
Your fiance is an ideal mate on a certain level. He has a beautiful bank account.
In all other respects -- personality, looks, commitment, values -- he is a loser.
The phrase can have a literal or idiomatic meaning. Without context, I cannot say for certain which meaning is intended, but I would guess the idiomatic occurs much more frequently.
Literally, it would mean on a certain floor (e.g. on the fifth floor).
Idiomatically, this means that one is engaged in the activity to a certain degree or when looked it in a certain frame, that's the purpose of one's activity. To make that a little more accessible, if I said that I am here to answers questions on a certain level then I would mean that if you look at my purposes in some respect that's why I answer question on English.se. Alternately, I could say I answer questions because of curiosity ... on a certain level.
As such, the idiomatic usage is that if you narrow down what's going on to one thing or one type of thing, then it would be that.
Probably with your example, the speaker is doing what appears to be many things but in answer to the purpose of what they are doing explains that they are looking for their sister -- even if the way they look involves things like say playing video games or earning money working as a bartender or anything else.