This is the 1305th most frequent Japanese word.
立
"立" means "to stand" or "stand."
Here, '立った' means 'stood' and is the past tense of the verb '立つ', which means 'to stand'.
彼は立った。
He stood up.
In this context, '立っている' means 'standing' and describes the state of the pen being upright.
机の上にペンが立っている。
There is a pen standing on the desk.
Here, '立派' is an adjective derived from '立' and means 'distinguished' or 'excellent', showing a different semantic usage.