Assembly is the code the machine actually runs, cleaned up just enough to be legible. On this site it looks like this:
0: li r3, 20
4: blrTwo lines, two instructions. Assembly is one instruction per line, executed top to bottom.
The numbers on the left don't count 1, 2, 3 — they jump by 4, and they're in hex. They aren't line numbers. Each is the address of its instruction: where that instruction sits in memory. That will matter later; for now, ignore the values and just know what they are.
Every instruction is a mnemonic followed by zero or more operands. The mnemonic says what the instruction does — load, add, subtract, return. The operands are what it does it to.
Take li r3, 20. The mnemonic is li, the operands are r3 and 20. li is short for load immediate, where "immediate" just means a constant written directly into the instruction. PowerPC puts the destination first, so the line reads as "put the literal 20 into r3."
r3 is a register — one of the small, fast slots inside the processor that hold the values it's working with right now. There are 32 of them, r0 through r31. So this one instruction drops the number 20 into register r3.
blr has no operands at all. It's the return: it ends the function and hands control back to whatever called it. Nearly every function you decompile ends with one.
There are a lot of mnemonics, and you'll meet them a few at a time. You don't need to hold them all in your head — hover any instruction in the diff and a tooltip explains it, with that line's own registers and values filled in. Use it as much as you want. The common ones will stick on their own soon enough.
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This is a simplification, of course. There's more to assembly than two instructions, and you'll pick the rest up as lessons need it. But it's enough to read your first target — time to match your first C function.