The main factor is what the grant covers. If you applied for money to hire a postdoc, then that postdoc would not normally be expected to do any lecturing (under EPSRC rules, for example, and probably most other councils under the UKRI umbrella), so your teaching still needs to be done by somebody and the university is not getting any money towards that. You would then be expected to keep your normal teaching load. If the grant covers, say, 50% of your time for 2 years, then the university gets to keep half of your salary, and can use it to hire a fixed term lecturer. In that case you might be able to negotiate some teaching reduction, although not all universities honour these kinds of buy-outs equally scrupulously.
Even in the second example above, you should not necessarily expect to really have your teaching load halved. For example with grants from UK research councils, the grant actually only covers 85% of what you applied for, and the rest is covered by your institution.
As xLeitix said, ultimately, it is totally OK for a new lecturer to be asking their colleagues carefully worded questions along these lines. If you do not start the conversation with an undue sense of entitlement, then your colleagues will happily help you evaluate the situation correctly, explain the norms at the institution (which can differ from place to place) and draw the right conclusions.