Generally, Summary looks at your committed spending (throughout the current Summary period) and removes it from your overall (non-committed) balance.
Example; if your Summary period is 01-June to 30-June and you started with (or deposited) £1,000 and have spent £856 so far - both committed and non-committed spending - and have a planned (committed) transfer of £100 to another account tomorrow, Monzo will report £44 is left until the end of the Summary period instead of the actual £144 remaining - because it knows there is a commitment to ‘spend’ £100 tomorrow.
Circumstances surrounding Summary vary greatly between users, but I hope the example helps.