The answer to your question really depends on where exactly the stiffness is that you are feeling and how significant it is in terms of either discomfort or impact on your ability to exercise.
For example. it is very common to have stiffness in the muscles of the lower back. This is usually due to chronic strain and fatigue, from posture while working (heavy lifting, for example, or even just sitting hunched over a computer screen all day long). When people are suffering from low back stiffness, there are often good therapies that can be used to improve the symptoms, including back strengthening exercises and better attention to good posture.
On the other hand, if your stiffness is more generalized throughout the body, then this might be a sign of an underlying medical condition that needs evaluation and treatment. For example, in older adults, there is a condition known as polymyalgia rheumatic, which typically causes stiffness in the muscles around the shoulders and hips. If you have any swelling around the joints in your body, then your stiffness may be a sign of a rheumatologic condition, such as rheumatoid arthritis
I suggest you start by getting an evaluation of your symptoms by your primary care doctor