I just ran across them in some tabs I was trying to play...
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Tremelo picking is picking one note then another really fast. Mute note could either refer to a palm mute or a muffled string.
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Tremolo picking is picking a note/s on one string really fast, by picking up and down rather fast.

Mute notes can either be marked as -x- on the tabs, and that just means to lay a finger on that string anywhere (except for frets 5, 7, 9, 12, and 18, for they are harmonics) and pick/strum the string(s). If it's Palm muting, then you lay your palm just before the bridge, and pcik while your palm is there, thus creating a sort of 'muffled' sound.

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With tremolo picking, there is not normally a set number of times you hit the string. Just really fast until the note value is complete. Also tremolo picking motion comes more from your elbow that standard alternate picking would.
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^yes, you're not playing an actual phrase of notes (i.e. semiquavers, semiquaver triplets, etc) you're just playing as fast as you can on one pitch until the music says so.

and mute notes could either mean muffling the note (meaning it does not ring) or palm muting, whereby you rest you palm on the bridge touching the ends of the strings so they have a bit less bite when played.
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