Under the bonnet on the top left of engine there is a white container with max and min levels marked on and a screw cap on top.What is this for?Should l be adding water to this?l unscewed the cap and it looked rusty.Ca not work out what to add as even if you fill it up does not look like you can use the max min levels to get right level. Any help appreciated

If its a black cap with a triangle, a book, two arrows and F,O on it then its the header tank for your water. Mix anti-freeze 50/50 with water to top up. If its the other side of the engine bay then its the master cylinder for the brakes. Top up with dot 4 brake fluid.