Isotopes are one of the main methods of dating materials. Just one example is Aluminium - which exists in the 2 isotopes of Al-27 and Al-26.
The idea is quite simple. In the beginning you start off with Al-27 but gradually over time this becomes Al-26. The rate at which this happens is constant, so if you measure the relative amounts of the 2 isotopes then you have a good idea of how old the sample is.
Aluminium is more complicated because Al-26 actually then decays to Mg-26 (with a half life of 0.73 million years), but evidence from chondrites (the most common form of meteorite) suggests that Aluminium was found consistently around our solar system in its early days (at a ratio of 10%). This is important because it confirms that it is useful as a chronometer.
In turn we can again be confident that our universe is indeed very, very old.