A conventional bond is a loan. You lend money to a company or government. In return, they pay you fixed interest. This is riba, and it is prohibited.
Sukuk are the halal alternative. Instead of lending money at interest, you own a share of a real, tangible asset. Your return comes from the income that asset generates.
The UK Government issued a £500m sovereign sukuk. Investors effectively own a share of government buildings. The rental income from those buildings is what provides the periodic payments to investors — not interest. If the buildings generate more rental income, returns improve. If less, returns fall.
Sukuk are permitted because the return is genuinely tied to a real asset's performance, not predetermined interest. The investor takes on actual economic risk — which is the key distinction from riba.