Cheat Engine

[Freiza]

[Dark Byte]

first one helps people that don't understand that al is 1 byte long
second one is for people that know assembler

in short, it means the same

[Slugsnack]

firstly your first line is not correct. that instruction is not correct. presumably you meant to say :
mov al, byte ptr ds:[esi]

when the size is not implied for the operands ( can not be inferred by the assembler ) then it must be specified explicitly. in this case al is known by the assembler to be a 8 bit register so there is no need to write that. also the first line is what is commonly generated by disassemblers. assembler programmers tend to write :
mov al, byte [esi]

[TROLOLOLOLOLOLOLOLOLOLOLO]

What does that even do

[Slugsnack]

moves the byte pointed to by esi to al. in other words, it assigns al the value of the byte that esi is addressing.

[Freiza]

Does cheat engine follows masm syntax?

[Slugsnack]

Yes afaik