Without going into politics - impossible.
Without taking sides -
The Democrats who controlled the House, Senate and WH last year did not pass a budget for this year. (it was higly political - I'll decline comment)
So now, without a budget, operations have been funded by Continuing Resolutions. The last one ran out today (actually at midnight)
The Republican House has proposed CR's and a rest-of-year Budget that makes some minor cuts to the $1.5 Trillion portion of the budget which they can affect.
The Senate will not agree to that .05% of cuts.
The President has vowed to veto the latest proposal even if it passed the Senate - which it won't and the Senate won't even schedule a hearing to consider it.
So the Congress and WH are deadlocked and the funding ends at midnight.
In a shutdown, the President has the authority to designate who is Mission Essential and who is not. In the past when this has happened (and it has from time to time - this is nothing new. We joked at one time during the Carter years about "working for peanuts" and really being in an "All Volunteer" military) the military was off-limits. Not this time. The President has specifically included the military into the mix. The active duty types have to report for duty, but may not get paid on time, until the budget is settled. Programs that are not fully funded, are stopped. Military operations will continue, we just won't pay the troops carrying out the operations.
Of note, the DIC payment is specifically suspended during the shutdown! (this will make sense only to US military personnel, if you don't get it, don't worry about it)
That, in short, is what is going on right now.
I've left the specific issues off in order to stay out of the political issues being fought over.