In typical patio furniture you would find the legs of the furniture damaging the floor and for that you will need the round or the square glides. The glides fit on the bottom of the chair tubing and prevent the bottom from scratching the floor. If you see that the glide has worn out and the chair is damaging the floor then it is time for going in for the replacement of the glides.
The inserts are used for closing the open ends of the tube to prevent the corroding of the inner side of the furniture. That is why check regularly as to if the glides are in the chair or the table and the metal from inside has not begin corroding. If you feel that corroding has begun then replace the glides with the new ones.
End caps or the disk caps seal the end of the aluminum chairs or the regular teak chairs. These get worn out by regular use to buy new ones once you see the end caps going bad.
If you have patio umbrellas then you need to check for the tightness of the rib clips and the ease with which the umbrella opens. If you feel that the umbrella is not functioning properly then check the rib clips, adjustment brackets for the adjusting the height of the bracket
Most of the patio furniture that you will buy will have the vinyl tires at the end. These also get worn out with use so check these and replace them to prevent damage to the patio furniture.
Apart from these the main things that you need to know about repair are other patio furniture replacement parts like the slings and the sling rail caps which are used in the patio chairs. Slings get worn out very easily with regular sitting. Slings should be replaced regularly for that special new look of the patio chairs.