As the previous posts state, the short answer to your question is that strength training is training where your muscles are working against a resistance. Even though a lot of people see strength training as being the same as weight-lifting, it isn’t necessarily the same. Strength or resistance training is a broader concept in that it can literally involve any kind of resistance (e.g. resistance bands, elastic tubes, body weight etc.).
Strength training can involve several different methods, depending on your goals. For example you will train differently if you want to build strength endurance (being able to perform work that requires a certain level of strength over a period of time) than you would if you simply want to increase your raw or one-repetition maximum strength (training to lift or otherwise move the most weight you can).
Also there is a difference between “relative” strength (a person’s strength relative to their size) versus “raw” strength (simply looking at how strong a person is irrespective of their size).
So as you can see, while strength training itself is a fairly simply concept how you actually train to improve your strength will depend on several factors including the reasons why you’re strength training. I hope this helps!