It's a bit like "give an estimate", but on a larger scale.
So for example if you want to have your house extended, and you have some plans about the sort of building work you want done, you can contact a builder and ask them to come and evaluate the work and prepare you a detailed costing of these works, including the overall cost. This is "an estimate". (This word obviously has other meanings in other contexts...)
Similarly, if for example you are a city council, or a government department, or maybe a large company, and you wish for some large project to be costed and evaluated (e.g. a new public building, new software...), often several companies might be asked to "submit tenders", with their evaluation of the costs. Then the company with the best proposal is usually selected by the commissioning organisation.
So an estimate would probably be one, or possibly two pages with the list of items and costings, whereas a tender might be a file running to many pages, with details of the many aspects of the project being tendered for.