Higashimaru is a proper noun in this case, probably name of the company. Kabayaki is grilled eel, usually grilled over charcoal. "Tare" is a noun for sauce, "no" indicates ownership or close association.
Inverting the grammar so it makes sense to English speakers would be "Sauce for Kabayaki".
Another use would be to make rice balls, baste them lightly with this sauce and then grill the entire riceball (preferably over charcoal), until it has light brown score lines.