Electrolytes

Electrolytes are salts with electricity conductive properties. Throughout our body tissue, fluid, and blood, electrolytes conduct charges that are essential for muscle contraction, heart beats, fluid regulation, and general nerve function.
-- Brendan Brazier author of The Thrive Diet (from Thrive, the vegan nutrition guide to optimal performance in sports and life, page 286)

Electrolytes consist primarly of the minerals chloride, calcium, magnesium, sodium, and potassium. The balance of electrolytes in our bodies is essential for normal function of our cells and our organs.
If the electrolytes in our system are low the absorption of water will be inefficent and will eave us feeling dehydrated, when we drink too much water we can also flush the remaining electrolytes out. So achieving balance is important, drinking adequate (but not too much) pure energised water and replenishing our salts regulary.
A very good source is dulse (purple seaweed), which comes in flakes from your health food shop and can be easily added to your drinking water, it's a bit like a purple confetti party in your drink bottle.
They are also present in other seaweeds, Celtic sea salt, Himalayan Crystal salt, citrus fruits, and homemade 'sports' drinks (which unlike commercial ones don't have unneccesary refined sugar and chemicals).

In Brendan Brazier's book he offers a few delicious and nutritionally considerate recipes for sports drink alternatives. I often just add dulse to my water bottle, or if really dehydrated I drink warm salty water with a squeeze of lemon and a tiny bit of maple syrup (for glucose). Young coconut water (if you can access it) is an incredible source of electrolytes.

--
Electrolyte Sports Drink with Ginger (thrive, p122)
'the young coconut water provides the electrolytes, while the agave nectar delivers an easily digestible form of slow release carbohydrate. The Ginger helps minimise inflammation.
1 lemon
2 cups of water / coconut water
1 tbs agave nectar
1/2 tbs grated fresh Ginger
sea salt to taste
blend
--
Lemon-Lime Sport Drink (thrive, p123)
juice 1/2 lemon
juice 1/4 lime
3 dates
2 cups water
1 tbs agave nectar
1 tsp coconut oil
sea salt to taste
process in a blender until smooth