Alternative tenders are proposals from suppliers who choose to offer an alternate solution to what is specified in the Request for Tender (RFT) by an organisation seeking tenders.
In simple terms, it means offering something different to what is being asked in the RFT. For example, if you are responding to a Request for Tender to build a house that specifies the roof to have tiles and you think that metal roof such as colour bond will be better, then you would submit an alternative offer that will have colour bond roof.
Both of them will achieve the same outcome, but the alternative tender will not be what is actually specified in the Request for Tender. There may be reasons for them requesting tile roofs, perhaps all other buildings around them are tiled roof and they want to keep it consistent.