The cartoon shows a man lowering a barrier with the words 'Keep Out' on it (a clear metaphor for Brexit). However, a Scottish man (note the kilt, tartan, and tam o'shanter) tells him 'Hold it!'
A majority of Scots (and Welsh, and Northern Irish) want to remain in the EU, so if the UK (i.e., the English) votes to leave the EU, that could trigger a second Scottish referendum and lead to the break-up of the UK. In which case, Scotland could apply to rejoin the EU (though it's not a foregone conclusion).
If you tell someone to 'Hold it' you want them to stop doing what they are doing.