Basically,
say the government has £100 in tax income this year.
And a £100 pilled up in the bank of england.
And plans to spend £250.
Someone points out they only have £200, so asks "where is the other £50 coming from. Are you going to borrow it?"
The government says "no need, we have £150 pilled up now".
"But where did that extra £50 come from?"
The government says "We printed it."
Sounds easy, but the consequence is inflation, because the government still only really has £100 in 'real terms', and the £50 just came from thin air. So now each pound note is only worth 66p of what it was before.
A big problem would be imports, because other governments would know you don't really have £150's worth of assets (like gold) to back that £150 of paper money, just £100's worth.
So whilst your £100 might have bought you 100 euros before, now £150 still only buys you 100 euros.
If you only printed £1 more you would probably be OK - and governments do that all the time. In fact they also do the reverse, and destroy £1.
But as soon as you start printing far too much, inflation sets in, confidence falls, and before you know it you have to print even more, and so it goes on. Before you know it, you need £1m to buy something that once cost £1. All because the government tried to print its way out of a shortfall.