To follow someone around; or, the person who follows someone around.
1) You're going to the new coffee shop : Do you mind if I
tag along ?
2) My little brother is such a tag along - he follows me everywhere I go !
A 'tag' is a piece of paper that hangs from something - for example, there is a 'tag' on a new
pair of pants to tell you how much they cost. 'Along' means to go with somebody but to follow
their lead, to 'hang' on them as they go about their business.