I came across this sentence
He has been by my side, my rock and shoulder to lean on literally, every step of the way, caring for me and Erin, holding my hands, holding me and loving me.
This is from a person's autobiography. I wonder if this sentence "my rock and shoulder to lean on literally" needs a verb like "my rock and shoulder lean on literally". What' the meaning of "to lean on" in this sentence? What's the usage of this infinitive?