A bound morpheme is a morphological element that can only appear as a proper subpart of a word, i.e. an element which cannot function as an independent word or free morpheme.
The English word agreement contains the nominalizing affix -ment which is not a word in its own right, and therefore -ment is an affix of English. A form like mit (as in permit, remit, commit, etc.) which is not an affix but a root (since affixes may attach to it, forming a word) also cannot occur freely in syntax and may be called a bound morpheme for this reason.