These five buttons are for quickly editing the envelopes of the currently selected notes. You can hover over them to see a longer description of what they do.
To view note envelopes, make sure that the [~] button in the bottom left corner of UTAU is pressed. These shapes represent the volume of the notes over time.
ACPT (Apply automatic parameter adjustment) will copy the preutterance and overlap values from the voicebank's configurations into the note's properties. Sometimes, when the previous note is very short, the STP (starting point, a value added to offset), preutterance, and overlap will need to be automatically adjusted in order for all of the notes to fit together and all stay in time. Those adjusted values will be applied to the note properties instead of the original values in the oto.ini.
P2P3 (Set crossfade envelopes by p2 and p3) will adjust the envelopes so that the beginning and end of each note will fade in and out, rather than start and stop abruptly. By fading the volumes between the notes, they can blend together more smoothly during the part where the notes are overlapped. You will notice here that the first and last points of the envelope are in the bottom corners. If you decide to edit the envelopes, this makes it easier to make the volume cut off entirely at the beginning or end.
P1P4 (Set crossfade envelopes by p1 and p4) does the same thing as P2P3, but the first and last points are used for the top corners of the envelope. If you decide to edit the envelopes, this means you have more points in the middle for creating volume variations throughout the note.
OPT (crossfade optimization) will adjust the STP of each note slightly based on the previously played audio, so that each note blends together a bit more smoothly.
RESET (Reset envelopes in the region) will reset the envelopes for all selected notes to default. If you used ACPT or OPT, or if you're using a premade UST where someone else might have used ACPT or OPT, and especially if you're now using a different voicebank, you should also check the region properties for the notes to make sure that Preutterance/Overlap/STP are erased.