This is the 4497th most frequent Russian word.
вчерашнего
"Of yesterday" or "yesterday's."
In this sentence, 'вчерашнего' (yesterday's) is used as an adjective to describe the noun 'хлеба' (bread), indicating that the bread is from the previous day.
На столе остался кусок вчерашнего хлеба.
A piece of yesterday's bread remains on the table.
Here, 'вчерашнего' modifies the noun 'разговора' (conversation) as an attributive adjective, specifying that the conversation happened on the previous day.
Он был расстроен из-за вчерашнего разговора.
He was upset because of yesterday's conversation.
The word 'вчерашнего' qualifies 'вечера' (evening) and is used to denote that the evening in question is the one that occurred the day before.