Microsoft documentation defines SQL Server blocking as " blocking occurs when one session holds a lock on a specific resource and a second SPID attempts to acquire a conflicting lock type on the same resource" - sounds easy enough. But troubleshooting and resolving a block leading to a performance problem can require different approaches .
How to monitor Blocked Processes with SQL Alert and email sp_whoisActive report
How to Use sys.dm_tran_locks to find requests waiting for an exclusive lock
Blocked Process Report and how to read
Database backup does not cause blocking
How to Find queries causing Lock Timeouts per second
LCK_M_IS sql wait type - how to reduce
A guide to Understand Key wait resource in Blocking
How to find Open SQL Transactions
How to Identify Page Latch contention
The simple guide to SQL Server Cached blocking history with sys.dm_db_index_operational_stats
How to detect and troubleshoot blocking and deadlocks
SQL Blocking script ==>This is how I used to look for blocks - adding for historical purposes