Effectively, it is a large array of variables.
Memory is accessed by supplied an address - effectively an array index.
Data may be fetched (read) or stored (written) to a particular memory address.
Most modern computers store data in multiples of 8 bits - bytes.
Most modern computers are byte-addressable - each byte has a separate memory address
Often multiple bytes can fetched/stored in a single operation.
For example, a computer might be able to fetch/store 1,2,4 and 8-byte quantities from memory.
A commonly used number of bytes may be termed a word.
For example, if 4-byte values (32 bit) are commonly fetched/stored this may be termed a word.
This terminology varies - on other machines 2-byte quantities may be termed words.