The Resolution of a photo refers to the number of pixels, ie dots of colour, which comprise the image. Your camera will determine the maximum resolution of your photos though most cameras can be set to take photos at a lower resolution than the maximum. You can also use photo editing software to reduce the resolution of a photo after you have taken it, eg to save space in an email or on a webpage.
A high resolution photo will have more pixels than a low resolution one and will produce a better quality image particularly when printed at large size. It will also take up more space in your computer or on your camera memory card.
For example, the photo below is 2560 pixels horizontally and 1920 pixels vertically and occupies 2.1 megabytes:
... whereas the following photo is only 640 * 480 pixels and occupies only 104 kilobytes.
If you look carefully you can see that the second image is less sharp than the first one and this difference would certainly show up if printed. However for viewing on a computer screen the differences are far less significant.
Your digital camera will have a maximum size of image which it can take, thus a 5Mp camera can take photos with a resolution of up to 5 mega pixels. Y ou can usually adjust the settings on the camera to reduce the resolution of images taken to save space on your camera memory card.
Once you have taken a photo and donwloaded it on to your computer, you can use photo editing software to reduce the resolution of the photo in order to reduce the file size, eg for including in an email. However, once you have taken a photo you cannot increase its resolution.