What does the hash (#) value associated with the ARM LDR instruction mean?

I'm trying to debug a crash I am experiencing in my application. The stack trace is pointing to an LDR instruction with the following format (thanks disassembler):

LDR R3, [R0,#4]

My question is in regards to the source component. What does the #4 in the second parameter mean? I'm assuming it is some kind of offset, but I haven't found documentation supporting that for the LDR instruction.