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(sneak breathe) is referring to the fact that you've got a very long slur/tie thingy and that you will have to sneak breaths along the way. i.e. play for as long as you can in one breath, and keep going if it wouldn't be smooth to take a breath where you are....or just plan it out so that you breathe in places that would be less noticeable than others.

the tremolo is basically like a trill. the two notes tell you which notes to alternate between, the note values of those two notes(like the whole notes at the beginning) tell you that you'll be doing the tremolo for that long. the LINES tell you how fast to do it. in this case, you have three lines so you'd be doing 32nd notes.

so you can see how it's like a trill, in that it's going back and forth between two notes. it's different, though, because it's a controlled, specific speed designated by the lines. (if it were two lines you would only play the speed of 16th notes).

tremolos can cover large intervals, such as thirds, fourths, fifths and, theoretically any other interval...though it would be ridiculous to expect anyone to tremolo between a low c and a high c.

tl:dr
like a trill, but with controlled speed between the two notes indicated.

think of it this way, those lines are like the flags on notes like 8ths, 16th, 32nds, etc. it's a way to write the same interval over and over without actually writing each individual note. it's sort of like the "repeat one measure" sign.