The default src attribute acts as fallback for a default image size for browsers that have no support for srcset:
The srcset attribute gives the browser a bunch of different sizes of the same image to choose from.
The name of the image doesn't matter, you could name them image-small, image-medium and so on, but the 375w part should match the real width of the image:
<img src="/images/image.jpg" srcset="/images/image-375.jpg 375w, /images/image-768.jpg 768w, /images/image-1440.jpg 1440w, /images/image-1920.jpg 1920w, /images/image-2560.jpg 2560w" >
Adding the sizes attribute specifies what image size to choose at a specific breakpoint and above (min-width) or below (max-width) it:
<img src="/images/image.jpg" srcset="/images/image-375.jpg 375w, /images/image-768.jpg 768w, /images/image-1440.jpg 1440w, /images/image-1920.jpg 1920w, /images/image-2560.jpg 2560w" sizes="(min-width: 1920px) 1400px, (min-width: 1600px) 1000px, (min-width: 768px) 700px, 300px" >
One thing to note here is that the browser will first calculate the device width and then look at the sizes attribute and choose the first matching condition.
If you set for example sizes="(max-width: 1920px) 1500px, (max-width: 768px) 600px, 300px", and the device width is 640px, the first matching condition will be max-width: 1920px, therefore it will choose the 1500px image width suggestion, rather than the 600 you most probably expected. The same can happen with the min-width if the order is not right.
Notice also that we are asking the browser to choose a 1000px wide image between 1600px and 1919px, but in the srcset sources, there's no image at 1000px. Here, the browser will choose the image with the exact size, if there is one, or the closest one if not, but higher in size than specified, in this case, 1000px. Therefore, the 1440px will be selected.
It can take a while to grasp how this works, but it's worth taking the time to understand it.