An arpeggio is a chord or chords that is broken into a sequence of notes. The sequence can span from one to multiple octaves and most modern synths allow you to choose the order in which the notes are playing, so they might go up, down, play randomly and so on.
Synth arpeggios can be very diverse – short and plucky, full of sharp transients, all the way to softer, mellow and atmospheric – which means deploying multiple mixing techniques. As with most leads there is always room for an EQ to help tidy the sound up, cutting unnecessary low end or reducing a resonant frequency. We can also emphasise a frequency band, like a high shelf boost, to help the sound be more up-front.