What specifically is a "register plus immediate" addressing mode?

My question arises from the following problem I have been trying to do for a while.:
"Why does adding addressing modes like register plus immediate to an ISA tend to improve performance?"

Question

What specifically is a "register plus immediate" addressing mode?

My question arises from the following problem I have been trying to do for a while.:
"Why does adding addressing modes like register plus immediate to an ISA tend to improve performance?"

s3a · Answer

Solution to the problem:
"Adding additional addressing modes to an ISA tends to improve performance by reducing the number of instructions required to compute addresses. For example, if a data structure contains four data words, register plus immediate addressing can be used to access all of them, with only one address computation required to change the pointer to point to the next data structure. If an architecture only provided the register addressing mode, an ADD instruction would be required to calculate the address of each element in the data structure. "