It's a very affectionate and emphatic way of expressing love. The phrase combines "te quiero" (I love you) with "tanto" (so much) and "mucho" (a lot).
While grammatically correct, the phrase is a bit redundant. The use of both "tanto" and "mucho" is common in everyday speech, but it's often considered more natural to say simply "Te quiero tanto" ("I love you so much") or "Te quiero mucho" ("I love you a lot").