This is a base for an exterior umbrella. The screw isn't threading to the base anymore so I can't tighten it.
Is there an inexpensive way to "rethread" screws?
Right now, it just spins.
Are you sure that's the right bolt?
3/8 UNC and
M10-1.25 look kind of the same but are not compatible.
Oil, serioiusly just adding some oil can work wonders.
A new bolt can help too. bolts are cheap.
To rethread the hole you cau use a thread cutting tool called a tap, or since it's just rust in the way you can probably use a hardened bolt with a slot cut in in as a cheaper alternative, use lots of oil.
That looks like a bolt that locks the umbrella pipe to the stand pipe, so wind does not do a Mary Poppins.
The cheapest way will be to drill right though both pipes and use a rod/bolt to lock them together.
A small hole though the end of rod/bolt with a hair clip pin will keep it from falling out.
Never worry about threads again.
A line painted on both pipes will help align the holes when placing the umbrella back in.
Apart from replacing a damaged bolt, if applicable, or drilling out to a larger size and rethreading to take that size bolt (as already suggested) you can (for somewhat more money) drill out a stripped hole to a different specific larger size and insert a repair threaded insert to restore the original threaded size of the hole - this is commonly needed for things like spark plugs where there isn't a "larger size bolt" that will work.
HeliCoil® (I've used them, not associated, no endorsement) is one major brand of such inserts, and you can get a complete kit with the inserts, a proper size drill bit, the proper tap for the insert and a tool to put the inserts in with.
In this case, the "use a larger bolt and re-tap to suit it" option is probably more sensible (unless the bolt, rather than the nut, is stripped, where simple replacement and possibly chasing the nut threads, if damaged, with that size tap might work.)
It depends on what exactly you mean by "can't tighten". Does the bolt jam or does it spin without making progress.
A "tap" cuts an internal thread, A "die" cuts an external thread. Taps and dies are not horribly expensive.
If a thread is damaged or contaminated, such that it jams up then running a tap/die over it can "clean it up" and make it functional again.
On the other hand if the bolt is spinning without making progress then that likely indicates that material has been lost, either through corrosion or mechanical damage. A tap and die can't put back missing material.
For the bolt I would suggest just replacing it, it's not valuable enough for any repair attempt to be worthwhile.
If the thread in the hole is too damaged to use, then you have a few options.