TO LEAN intransitive verb 1. to lean means to incline, to bend from a vertical position. [Trees are leaning eastwards from a heavy West wind.] 2. to lean as a second meaning is to cast one's weight to one side to get support [As I am tired, I am leaning against the wall.]
TO LEAN transitive verb to rest, to support something (on, against..) [Lean your bike against the wall; lean your head on my shoulder.]
Both functions of the verb are working with BACK which points out the direction of the leaning. to lean back = to bend backwards (one's body) [I leaned back to dodge the slap; the car seat leans back as you pull that lever.] to lean something back = to bend it backwards. [Lean yourself back on the armchair.]
Note that to lean has also a figurative sense, to incline in opinion, in taste... but I don't think it gets used with "back". To lean has other meanings, for instance to lean an engine (make the mixture less rich). Look it up in a dictionary. [CW]
lean or learn??
Do you mean we can lean on you for a learning support?