what does "fluids" mean in the following setence?

Well, to be more sepecific, intravenous injection usually implies a quick administration of a substance intravenously with a hypodermic needle and syringe. Intravenous administration, or IV usually involves a hypodermic needle being inserted into and kept in a vein for a much longer period of time.

The sentence refers to IV administration or IV therapy.

Yes, that is what it means. As Infrablue points out, it means putting a needle in (usually in the forearm) and leaving it there. The most common way to keep up a patient's fluids is with D5W--dextrose 5% and water.

However, in addition to I-V injection, there can also be an NG (naso-gastric) feeding tube that administers liquid nutrition and meds (gravity fed), as well. It might depend on the quantity of liquids or meds being fed. As I understand this, N-G tube is used for continuous feeding.

http://en.wikipedia.org/wiki/Nasogastric_intubation

However, judging from the context, it may mean they started treatment with an I-V injection and then switched over to an N-G tube after 2 weeks.