FFO stands for funds from operations. Net earnings for a property owner get pushed down by depreciation, which is a bookkeeping charge for the building wearing out. It's a real expense in the long run, but it's not cash leaving the company this year, and well-maintained real estate often holds or gains value while the books say it's declining. So earnings understate the cash a REIT actually generates, and a price-to-earnings ratio for a REIT looks strange for that reason. FFO adds depreciation and amortization back and strips out gains from selling buildings, which are one-time events. You'll also see AFFO, adjusted funds from operations, which subtracts recurring capital spending like re-roofing and tenant improvements. AFFO is closer to what could actually be paid out.
On yield: dividend yield is the annual dividend divided by the share price, so it goes up when the dividend rises or when the price falls. A 10 percent yield is usually the price telling you something. The check is the payout ratio against AFFO. If a REIT pays out more than it generates, the dividend is being funded from borrowing or asset sales and can be cut. Look at the last several years of dividend history too.
Broad equity REITs typically yield in the 4 to 6 percent range, so 6 percent isn't alarming by itself. There's no clean line, and the sector matters as much as the number. Mortgage REITs run structurally higher yields because they own loans rather than buildings, and their share prices behave very differently when interest rates move. If your high-yield list is mostly mortgage REITs and office, you've found the two explanations.