Non-blocking is a quality in switches that tells you that a switch doesn't lose frames unless a destination/egress port reaches its maximum bandwidth. That knowledge provides peace of mind and simplifies network design.
In a blocking switch there's a (sometimes unknown) total bandwidth that you cannot exceed. The bad thing is that it's not very obvious and might be really hard to detect that you're hitting the backplane forwarding limit and losing frames.
While small, 1U switches are all non-blocking today (except for really high-speed ones), larger chassis switches usually have a maximum backplane bandwidth and, more specifically, a limited module-to-module throughput that you need to know about when maxing out your network. Not all vendors properly document those limits - and not all network engineers have them on their radar at all times.