Parallel
noun, verbal noun, の adjective
Line up and go together. If lines did that, they'd be parallel.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
複数の言語を並行して勉強することは、ひとつだけを学習するよりも効率的だと聞いたことがあります。
I heard that studying multiple languages at the same time is more efficient than only studying one.