the simple explanation is like this :
the company is needing money for operational things such as exploration, drilling, etc.
they are running out of cash, and now trying to borrow money.
in this case (MXR), they are offering shareholders to buy renounceable issue share at the price $0.20c for every 1:2 shares they are holding.
if they buy, then for every 5 shares they will get a 1 bonus options exercise at 20c each.
why sp down? because at the current price of 33c, 20c issues is consider low. almost 40% discount.
also with new shares later to be allocated, the shares on issue will be almost double than now.
hope this explains.
regards