A ‘mast’ year is when there is a super abundance of nuts and fruits such acorns, beech nuts, hazelnuts, horse chestnuts, sweet chestnuts, apples etc. Click on this link for the definition: https://www.woodlandtrust.org.uk/blog/2017/10/autumn-is-back/
My experience walking around this year in late summer and early autumn is that this is definitely a ‘mast’ year. It has been difficult to walk down some steep footpaths in the woods due to the amount of nuts on the ground.