If you are in London right now on a holiday, or are coming very soon, you will know or should know that we are in a phase of tube (Underground) strikes.
These seem to typically start at 12 noon on one day and finish at 12 noon the next. This means that trains before the start will be affected, as they need to be moved to a correct place, and trains after the end will still be affected and will be slow to get back to normal.
Some stations may be closed, some lines or parts of lines too, and services may be reduced in frequency. First and last train times may also be affected. It may be at very short notice.
If you are here, avoiding the tube is advised, for several reasons. They may just not be running, and if they are, may be very crowded.
If you are in central London sightseeing, bus or walk (the weather is nice, if a little chilly) are better bets. Be aware that there are some bus strikes too at times.
Apps like CityMapper will give up-to-date information, and the TfL website will give a good overview of the whole disruption: https://tfl.gov.uk/campaign/strikes