Screen resolution is the number of pixels (colored points) on your screen. Those numbers, which are expressed as a pair of horizontal and vertical numbers, determine how sharp any particular image can appear. If you try to display an image that contains more pixels than your screen, it will lose some detail but that may not matter. It depends on what you're accustomed to.
There are many different screen resolutions used in laptops but only two are currently relevant: FHD and UHD.
FHD (Full High Definition) is 1920 x 1080 pixels. It's typically advertised as "1080." It's probably what you already have on your current laptop, monitor, and TV. All software, including Windows itself, scales properly on an FHD display.
If you have an external FHD monitor, the type of cable you need is determined by the interfaces the monitor has.VGA (analog) is the oldest interface and is not recommended. DVI-D (digital) works fine but HDMI (High-Definition Multimedia Interface) is better and easier to connect. DisplayPort is the newest and best interface. You can get adapter cables if necessary.
UHD (Ultra High Definition) is commonly known as 4K but the actual number of pixels varies from one manufacturer to another. There is no industry-standard definition. It's 3840 x 2160 pixels on Dell Laptops.
Anyone trying to sell you a new laptop will tell you to get 4K but it's not necessarily good advice. The resolution you should buy depends on what you use your laptop for and what you may use it for in the future. It's a waste of money to pay for 4K unless:
your computer's graphics card can display 4K content
you have 4K content such as 4K or Blu Ray optical disks or a streaming video source that provides 4K content
Go to local stores and ask a salesperson to give you a demonstration of 4K video content compared to DVD video content. If he can't do that get a different salesperson or go to another store.
At this point in time almost all software products scale properly on a 4K display but a few may surprise you. The main screen may look perfectly fine but a dialog box may look like a postage stamp. The Windows magnifier is useful for those.
If you have trouble reading text, go to Settings -> Ease of Access. You can adjust the size of text or everthing on the screen. I generally set text on a 4K monitor to 200%.
If you use an external monitor it will have to be 4K as well. If you want to do 4K gaming at high settings with a high frame rate, use a DisplayPort cable but be very careful about what you buy. They're not all the same. Some laptops have a Mini-DisplayPort output and you can get an inexpensive adapter for any type of cable.