OK. To borrow an earth, means to run a wire to bring the earth to a fitting, but not necessarily by the same route as the original live and neutral feed. In the limit, you connect one end of a wire to the last nearest place known to have a good earth contact, and then thread that wire through to the new fitting, using bamboo canes, curtain wire drain rods or whatever to pull it through to minimise the number of holes you need to cut and later fill. The end of the wire at the new fitting is the 'borrowed' earth. So called because it comes from another place.
This is not a shining example of good practice, but is certainly better than the alternative of leaving the earth contact on the new fitting disconnected. Suitable green/yellow covered wire can be found at most electrical wholesalers. If many fittings need this treatment than it is much better to renew the wiring properly.
However, you may find that the bare (but green sleeved) wire you can seen nestling along side the red one is the earth you need but it has been cut back out of the way as 'unneeded' , in which case all you need to do is extend the end of it, with a joint plus short length of new wire, or if you can just pull a bit more cable through if there is some slack, and remake the ends off.
However, how old is the cable?, (or when was the house built?) If the other lights do have an earth, then it probably gets most of the way there already.
BUT as already indicated, if the house is a mix of earthed and non-earthed fittings, it is possible the wire is there at the fitting, but it never makes it back to the main earth terminal, being cut somewhere else as unneeded. If you have meter with an 'ohms' range, this is easy to check, as there should be a low resistance (only an ohm or two max) between the wire that should be earthed, and something you know for certain is (like the main earth terminal, or an earth of a nearby socket.)
regards,
M.