Shelf registration (S-3, F-3)
A shelf registration lets a company offer securities later, in pieces, without a new full registration each time. Filing one says the option exists; it doesn't mean any shares have been issued.
Prospectus supplement (424B5)
A 424B5 describes a specific offering under a shelf: for example 15,000,000 new shares, or an at-the-market program of up to $500 million in new shares issued over time through a broker at market prices.
Dilution
New shares split the same company into more pieces. If a company with 100 million shares issues 20 million more, each existing share is a smaller slice (1/120 million instead of 1/100 million), though the cash raised also goes to the company.
Reverse splits and halts
A 1-for-10 reverse split turns every 10 shares into 1 at ten times the price; the total value is unchanged at that moment. A trading halt pauses trading in a stock, for example while news is pending or after a sharp move.