To give somenone false hopes or to make someone believe her/him you are head over heels for that person when you know for a fact you don't give a damn about it, but you keep it under wraps because you're either scared of hurting her/his feelings or you don't wanna be alone. Sometimes that guy or girl winds up marrying someone else (not always but happens)
John: Mary has really become a pain in the ass, but I don't wanna dump her. I'm so adapted to her company.
Sean: you're a fucking wimp, you should break up, you always string girls along!!!
John: after I hit on Monica!